Heating Services in Buildings

By David E. Watkins

Water based heating systems are efficient, flexible, versatile and offer many advantages over other heating systems. These advantages (fast response, good controllability, efficient zonal heating and largely silent operation) all require that initial design, installation, commissioning and maintenance be carried out to a high standard by competent engineers.

Heating Services in Buildings provides the reader with a detailed and thorough understanding of the principles and elements of heating buildings using modern water based heating systems. A key theme of the book is that there is little difference, in the approach to the design and engineering, between domestic and commercial installations. The author’s detailed but highly practical approach to the subject ensures there is sufficient information for students from both a craft background and those with more academic backgrounds to understand the material. This approach is complemented by straightforward, easy-to-use diagrams.

Heating Services in Buildings supports a range of educational courses, including degree level building services engineering; NVQ Level 4 Higher Professional Diploma in Building Services Engineering; City & Guilds supplementary heating course and the Heating Design and Installation Course accredited by the European Registration Scheme (ERS).  

• Language: English
• Publisher: Wiley
• Release date: Jul 7, 2011
• ISBN: 9781119971665

Book preview

Introduction to Heating Services

The broad term ‘central heating’ is used to describe many types and forms of heating, and some usage is totally misleading and inaccurate, through ignorance of the subject. This chapter is a basic introduction to the mechanics of central heating, which is discussed in greater detail in the following chapters.

If we examine the term, it implies a system where heat is produced from a central source and distributed around the whole building. The method of heat generation and distribution may vary with the type of heating system employed.

Central heating is sometimes referred to as space heating. To be understood fully, this must be described by its type or system arrangement, and may be categorised as being either full, part or background heating.

Full central heating may be defined as being a system of heating from a central source where all the normally habitable or used rooms/spaces are heated to achieve guaranteed temperatures under certain conditions. By today’s standards, all heating systems installed in residential dwellings and most commercial buildings should conform to this category, unless there are acceptable reasons for not doing so.

Partial central heating is the term applied where only part of the building is to be heated, but even then the rooms or spaces that are heated should still have guaranteed temperatures under stated conditions. This form of central heating would be a rare occurrence for a residential dwelling but not so uncommon for some commercial buildings, especially where part of the building complex is not normally occupied.

The term ‘background heating’ is used to describe a form of central heating whereby lower than normal or standard recommended temperatures are aimed at for the type of building involved. The term is sometimes used to refer to heating systems installed in buildings where the room temperatures are not guaranteed. This form of heating is unacceptable by today’s standards on both environmental and efficiency grounds.

It should be noted that, unless otherwise specified, full central heating should normally be designed to current regulations and standards and installed in a professional manner. In some instances, usually due to a specific use or financial reasons, the client may only require or specify partial heating to be installed, sometimes with the request that safeguards are included to allow the system to be extended at a later date to achieve full central heating.

Background heating, where lower than normal or recommended temperatures are aimed at, should only be used when specifically requested by the client for some reason. Even then, agreed temperatures should be incorporated into the design and guaranteed before any installation work commences. Under no circumstances should any heating system be installed without first agreeing specific room temperatures to be achieved when certain conditions exist. These conditions are discussed in Chapter 2.

Having understood the extent of the heating system and its classification, be it full, part or background heating, heating systems may be further divided under the headings of ‘wet’ or ‘dry’ systems. The terms wet or dry refer to the medium used to convey the heat from its source of generation to its point of use. Wet systems may be further classified by the piping circulation arrangement, with dry systems being divided into warm air and electric heating.

Figure 1.1 indicates the broad classifications of heating systems.

Figure 1.1 Heating system categories

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Heating systems can be sub-divided even further, but this will be explained in Chapter 21.

Wet Heating Systems

All wet types of heating systems employ a liquid as a medium to convey the heat from its source of generation. It is then distributed around the system to each heat emitter, where it transfers part of that heat through the heating surface of the heat emitters. Finally, the liquid is returned to the source of generation for the process to cycle continuously. The source of heat is commonly referred to as a boiler.

In all domestic heating systems, and most heating systems for other types of buildings, water is chosen as the medium for conveying the heat due to its low cost and being readily available. However, water does have the disadvantages of a low boiling point and high freezing point; it can also be corrosive to metallic materials and has a limited heat carrying capacity. The corrosive nature of the water can be reduced by water treatment, which is discussed later in this volume.

The temperature limitations and heat carrying capacity of water will have to be accepted unless we change the atmospheric conditions of the system, or we can change the liquid. Liquids known as ‘thermal fluids’ are available and have been used successfully on larger commercial type heating installations. They possess different properties to water, such as being less aggressive to common materials, having higher boiling points and lower freezing points, a greater heat carrying capacity than water and, in some cases, a lower viscosity. The merits of thermal fluids are much superior to those of water but are generally discounted for all domestic heating systems owing to their higher capital cost and not being readily available. They are also rarely used on larger commercial systems for the same reasons, but when conditions are right they can be considered attractive. The difficulty of availability can cause problems when replacement fluid is required immediately, following any emergency maintenance work. Thermal fluids have been used for domestic applications on limited occasions in countries that experience much lower temperatures than in the UK, as the lower freezing point of the fluid can be an important advantage when sub-zero ambient temperatures are experienced for prolonged periods with the heating system in a non-operating mode. They have also been employed as the heat carrying medium for some solar heating systems.

The purpose of the water used in heating systems differs from that used in domestic hot and cold water installations. In those systems, water is the end product or consumable item and after it has been used, it is discharged to waste. The water employed in a heating system is a non-consumable substance. It is the medium used to carry the heat required and, after it has transferred some of the heat, it is returned to the boiler to be re-used over and over again.

Dry Heating Systems (Warm Air)

Warm-air dry-type heating systems differ from wet-type heating systems insofar as the fluid employed is not only the medium used to convey the heat, but is also the end product. As the name implies, air is the fluid used to carry the heat from its source of generation, a warm air heater. It is then distributed, usually through a network of ducting, where it is arranged to enter directly into the room under controlled conditions to displace the cooler air. Finally, a mixture of the two is partly returned to the warm air heater for the process to be repeated.

Warm-air heating systems are generally disliked by many occupants of dwellings that have such systems installed, but this is usually because the systems are either not designed correctly, not installed correctly or are, in many cases, incomplete. This is mainly down to ignorance of the fundamental principles of warm air heating, which, if given the respect deserved, can be a very good form of heating. This work exclusively concentrates on wet-type heating systems since it is aimed at students and engineers in the plumbing industry.

Dry Heating Systems (Electricity)

Electrical heating systems may technically be classified as dry systems, but they do not employ a medium as they generate their heat at the point of use. For this reason, electrical heating systems are not included in this book, with the exception of heating systems that use electricity as the source of power to heat the water. Here they are classified as being wet or hydronic heating systems.

Supplementary Heating

This is a term applied to describe heating appliances, either fixed or portable, that are used to supplement the central heating system – either during extreme cold spells when the outside air temperature falls well below the base design temperature, or during the heating-off season in spring or autumn, when the outside temperature drops to below that considered comfortable.

Examples of such heating appliances include:

Radiant electric fires, portable and fixed

Oil filled radiators

Oil room heaters

LPG room heaters

Gas fires

Open solid fuel fires.

The list is not intended to be exhaustive, but meant to serve as a general representative selection of supplementary heating appliances.

2

Wet Heating Systems

Wet heating systems, commonly referred to as hydronic heating systems because they use a liquid as a medium, nearly always employ water as the medium to convey the heat from its source of generation, a boiler. This is rather a misnomer, as a boiler must be designed to avoid boiling the water, but is probably a leftover term from the days of raising steam. The heated water is circulated around the system, transferring part of its heat, and returns back to the boiler for the process to be repeated.

The water is fed into the heating system via a fixed piped connection to either a feed and expansion cistern, or a direct connection, as in the case of a sealed heating system. The water is allowed to enter the heating system slowly, thus avoiding creating turbulence, to fill it with all air expelled through the open vent, or by releasing it using manually operated air vents or automatic air release vents.

Water has many advantages as a heat carrying medium when used in hydronic heating systems; not least its plentiful availability. For this reason water is almost exclusively used for domestic heating systems.

Hydronic heating systems are classified by the following basic principles:

Temperature of medium

Pressure of system

Circulation method of medium

Piping arrangement for distribution.

The classifications are to a certain extent inter-related, as the selection of one of the basic operating principles has an influence on the selection of the others, which is explained in the following discussion.

TEMPERATURE AND PRESSURE

The classification of hydronic heating systems by the temperature of the circulating water exiting the boiler is closely related to the operating pressure of the system, and the two must be considered together. This is because pressure is required to maintain the water in a liquid form at high temperatures: as water will boil and convert to steam at 100°C at atmospheric pressure when measured at sea level, any increase in that pressure will have a corresponding increase in the boiling temperature of water. Likewise, any decrease in pressure below atmospheric pressure will have the effect of allowing water to boil at temperatures lower than 100°C.

Table 2.1 gives the temperature/pressure classification commonly used in the UK. The minimum pressures listed are those required to prevent the water from evaporating but should not be confused with their vapour saturation pressures, which are lower.

Table 2.1 Hydronic design operating water temperatures and pressures (UK practice)

*Account must be allowed for varying static pressures that would exist in a tall building.

It can be seen from Table 2.1 that water may be retained in liquid form when the operating temperature is above 100°C by pressurising it, giving all the advantages of a liquid and none of the disadvantages of a vapour such as steam. The method of pressurising the heating system is explained later in this chapter.

In contrast to the UK practice of temperature/pressure classification, in the United States of America the classification of heating systems differs slightly, outlined in Table 2.2.

Table 2.2 Hydronic design operating water temperatures and pressures (US practice)

It can be seen from Table 2.2 that the US has higher temperature and pressure classifications than the UK. However, in practice there is very little difference in the operating principles of hydronic heating system either side of the Atlantic.

Almost without exception, all domestic residential heating systems are classified as being low pressure and temperature (LPHW). It is considered safer to install heating systems using materials suitable for working pressures and temperatures below 100°C, therefore avoiding the potential hazard of flash steam occurring in the event of a pipe fracture or valve gland leak.

It has traditionally been the custom to design LPHW systems with a water flow temperature of 82°C and a Δt (temperature difference) of 11–12°C, giving a return water temperature of 71°C. More recently, the Δt has been increased in certain circumstances to take into account the requirements of condensing boilers that are influenced more by lower return temperatures than flow temperatures to function efficiently. This has a secondary effect on the increased sizing of the heat emitters, which is discussed in more detail in Chapter 8. Another situation where one should question the return water temperature and the flow water temperature is in heating systems employing underfloor heating sections that require the floor temperature to be limited to an acceptable level.

Low temperature heating systems may be further categorised as being either ‘open’ systems – where the heating system incorporates an open feed and expansion cistern and operates at atmospheric pressure, plus the static pressure created by the feed and expansion cistern at the traditional flow temperature of not exceeding 82°C – or sealed systems.

With a sealed heating system, the feed and expansion cistern is replaced by a sealed expansion vessel that allows the heating system to operate at a slightly higher pressure above atmospheric pressure and also permits the flow water leaving the boiler to have fractionally higher operating temperatures, in the region of 85–95°C.

If operating water temperatures higher than 82°C are selected for the heating system, then greater consideration must be given to the choice of heat emitters to be used, and all contactable heating surfaces such as traditional panel or column type radiators should be avoided so as to reduce the risk, scalding anyone who comes into physical contact with them.

Low water temperature heating systems are the most commonly used category of operating temperatures and pressures, suitable for all buildings ranging from small domestic residential through to very large and complex developments.

Medium temperature (MPHW) heating systems are favoured where a high heat output is desired so that smaller heat emitters and corresponding smaller pipe sizes can be used. The heat emitters must be of the non-contactable type, such as convectors, low surface temperature radiators and fan coil units. These systems are more suitable to commercial type buildings where the materials used are more robust than domestic low pressure type materials, and the system is more likely to be regularly serviced and maintained. This type of system in a domestic situation would be considered unsafe.

The use of high temperature and pressure systems (HPHW) is normally considered for use in industrial applications as some industrial processes require higher temperatures for manufacturing, or for developments that have a main central plant room that distributes the primary heat at high pressure and temperature to local plant rooms, which then circulate the secondary heat at a lower temperature. This arrangement is ideal for developments that are spread out over a large geographical area, and makes full use of more economical pipe sizes and equipment. As with the medium temperature systems, material selection and maintenance are critical factors.

CIRCULATION

Heating systems can also be classified by the method of circulation employed, i.e. either by gravity (thermo-siphon), or forced circulation by a pump, or a combination of both.

Full gravity heating systems have not been installed since the development of the glandless circulating pump. The practice of having a gravity circulation to the domestic hot water cylinder whilst the heating system has a forced circulation, which can have some merit when suitable conditions exist, is no longer permitted by the Building Regulations for residential dwellings, which unfortunately limits the design engineer in the options available. Even where the situation exists that the domestic hot water cylinder is located directly above the boiler at the optimum height, and the occupant’s needs are such that heating part of the system is not required for a great deal of the time but domestic hot water is, we are no longer permitted to use this method.

A fully forced method of water circulation for both heating and domestic hot water primaries is by far the most efficient arrangement in the majority of applications and gives freedom in the choice of plant equipment location, but this is not always the best option.

PIPING DISTRIBUTION ARRANGEMENT

Having discussed the temperature, pressure and method of circulating the water, the piping arrangement can be established. The different arrangements listed in Figure 1.1 form the basic systems for which there are numerous variations or modifications, but each may be categorised as belonging to one of the basic forms.

These arrangements each have their own advantages and disadvantages and the final selection should be made on the most efficient and economical method suited to each individual application. Also, a combination of any of the piping arrangements described may be used if it is considered by the design engineer to best meet the needs of the system.

The various piping arrangements depicted on the following pages have been produced to explain the operating principles of each system and are not supposed to be complete. For this reason most control elements and components have been omitted for the sake of clarity as these are dealt with in detail in Chapter 11. Also, the provision to include the means of producing domestic hot water has been included in each case, minus the controls element, to complete the piping arrangement: this may be by gravity primary circulation or by forced circulation. In most cases, either method may be used, unless noted otherwise. It is not the intention here to give the impression that either a gravity primary circulation or a forced primary circulation is the preferred option for satisfying the domestic hot water requirements, but just to show the different options.

ONE OR SINGLE PIPE SYSTEM

Of all the piping arrangements used for heating distribution, the single pipe system is the simplest. It consists of a single pipe main that extends from the boiler around the building as a circuit, or number of circuits, and returns to it with all heat emitters connected to the pipe by their own branch pipe flow and return connections.

Figure 2.1 illustrates the operating principles of the single pipe system and its limitations: a progressive temperature drop around the heating pipe circuit caused by each heat emitter returning its water back into the common circuit pipe. This has the effect of cooling the flow water available to other heat emitters being served by this circuit, which in turn results in subsequent heat emitters having to be oversized to compensate for a lower mean water temperature across the heat emitter.

Figure 2.1 Operating principles of single pipe heating system (non-condensing)

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To avoid heat emitters at the end of each pipe circuit having to be excessively large due to the decreasing mean water temperature available, heating pipe circuits should be limited to supplying water to a few heat emitters each, to restrict the mean water temperature across the heat emitter to no less than 70°C for non-condensing systems, or lower for condensing.

Another effect of pipe circuits suffering from excessive temperature drop is that the piping system on each circuit would also have to be oversized to compensate for the lower circulating water temperature.

The piping arrangement depicted in Figure 2.2 demonstrates that this need not be the case: if the branch circuits supply a minimum number of heat emitters, then the single pipe arrangement is just as suitable for larger domestic residential properties or commercial building applications, as the small domestic heating system.

Figure 2.2 Single pipe system for larger building with limited circuit Δt (non-condensing)

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The object in the design of this system is to limit the temperature drop across each pipe circuit so as to avoid having to significantly increase pipe sizes or heat emitter sizes to compensate, and so lose the lower cost advantage claimed by this system.

From the schematic layout depicted in Figure 2.2, it can be seen that if each piping circuit is limited to a reasonable temperature drop across it, and if each piping circuit is similar in its heat carrying load to each other, then the single pipe heating arrangement is suitable for heating system compositions in larger buildings. It can also be seen that the piping system is fairly evenly balanced in its heat distribution in order to achieve the temperature drop required from each heating circuit. This is accomplished by balancing the circulating piping system with the use of regulating valves when the heating system is being commissioned.

The primary flow and return to the domestic hot water cylinder in this illustration is in fact a two pipe arrangement. Lower temperatures may be selected for condensing heating systems.

The single pipe heating system benefits from the employment of special tees, known as ‘diverting’ or ‘inducing’ tees. These special fittings are designed to encourage a degree of flow into the heat emitter by creating a resistance to the flow between the flow and return branch connections, in the form of a pressure drop on the single pipe circulating main. This creates the conditions for circulation to occur through the heat emitter, as the resistance of this passage is less than that of the heating main.

The isometric layout illustrated in Figure 2.3 demonstrates the use of these diverting tees, whereby the up feed risers only require one diverting tee to be fitted on the return connection as the thermal head will assist the circulation, but the down feed pipes should be fitted with diverting tees on both the flow and return branch connections because no thermal head exists in this situation.

Figure 2.3 Application of diverting tees on single pipe heating system

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Diverting tees may be obtained in a copper alloy or malleable iron and are constructed with a venturi shaped restriction inside as shown in Figure 2.4. These tees are similar in design to ‘tongued’ tees that were commonly used on gravity heating systems.

Figure 2.4 Section through a diverting tee fitted on the return connection

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Figure 2.4 shows how the flow of water through the venturi of the diverting tee induces the flow from the return connection of the heat emitter.

Figures 2.5 and 2.6 show how the diverting tees are arranged and how they function for both upward connections to the heat emitters using one diverting tee on the return, and downward connections where diverting tees are employed on both the flow and return connections to the heat emitters.

Figure 2.5 Upward branch connections, standard tee on flow and diverting tee on return

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Figure 2.6 Downward connections, diverting tees on both flow and return

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Diverting tees have been successfully fabricated on site from standard capillary copper pipe fittings, either the end feed type, or integral solder ring type, using a standard tee, a spigot and socket straight reducing fitting, with the larger spigot end cut short but square, which is placed inside one socket end of the tee so that the reduced socket end protrudes past the branch of the tee. The cut spigot end of the reducer must be inserted into the tee so that the cut end has cleared the integral ring of solder if this type of capillary fitting has been used. The copper pipe is then inserted into the fitting and the capillary joint made in the normal way.

A 15 mm equal tee would use a 15 mm spigot end × 12 mm socket end reducer, and a 22 mm × 15 mm tee would use a 22 mm spigot end × 15 mm socket end reducer.

A variation of the single pipe system is depicted in Figure 2.8. If compared to the traditional one pipe system illustrated in Figure 2.1, it will be seen that the only difference is the method of connecting the circulating pipework to each heat emitter. The standard pair of radiator valves has been replaced by a single centrally located valve in the bottom of a specially manufactured pressed steel panel radiator that has a single female threaded centrally located tapping incorporated, together with a division baffle plate in the bottom main horizontal water passageway separating the two halves of the radiator. Figure 2.7 shows a cut-through section of this special three-way radiator valve.

Figure 2.7 Detail of central three-way radiator valve

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Figure 2.8 Single pipe central valve radiator connection system

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The central three-way radiator valve is connected to the radiator via a union radiator tail connection that has a central dividing wall along its length that lines up with the dividing baffle plate incorporated in the radiator, as shown in Figure 2.9. The valve is designed so that when it is fully open it will direct 100% of the flow through the radiator, and when it is closed, it will allow 100% of the flow to bypass the radiator through the valve. Any intermediate position of the valve control will direct an equivalent portion of the flow through the radiator with the remaining portion of the flow being allowed to travel through the valve bypass.

Figure 2.9 Arrangement of central three-way radiator valve and radiator

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This system heating arrangement was developed in Scandinavia, where it has found most use. Its main merits over the traditional single pipe system are that the three-way valve directs a definite flow of water through the radiator without the aid of diverting tees, and permits individual radiators to be isolated without affecting the flow to the remainder of the circuit. This arrangement, when it has been used with the heating piping installed either below or within the floor, forms a neater appearance with a minimum of pipework on show.

The disadvantages of this system are that the heat emitters are restricted to panel radiators made specifically for use with this valve, which requires confidence that they will continue to be available in the future for replacements, system extensions or alterations. Any thermostatically operated version of this valve yet to be developed would be required to conform to current requirements.

To summarise, the single pipe heating arrangement continues to be used for heating systems either in part, or in full in commercial buildings when the application is considered suitable, but is rarely installed in domestic residential properties. The reasons for this situation can only be guessed, but one theory is that heating systems installed in commercial properties are normally designed by engineers who approach each building on an individual basis, and consider the client’s brief, the architect’s design requirements, the structural constraints of the building, the budget available and the energy efficiency targets to be achieved. Only after having considered all of these important design aspects can a decision be made on the piping arrangement to be used, and, quite often the one pipe system meets all of the above requirements.

Unfortunately, this is not normally the situation regarding domestic residential dwellings. A high percentage of installers are also the designers of the heating system and it is quite common for them to install their favourite heating system arrangement in every property because they are familiar with it, regardless of the size or configuration of the building, or the client’s own personal requirements.

This text has demonstrated that the single pipe heating system, if selected correctly and after careful design, such as limiting the temperature drop across each piping circuit, will achieve a simpler, less expensive heating system and is suitable for many applications in domestic residential dwellings where normally the installer would not consider it.

It should be pointed out that not all installers of domestic heating systems have this blinkered approach and with ingenuity some very successful one pipe heating systems have been achieved, but for the majority, those who only use one type of heating arrangement regardless of the building layout, more training is required.

TWO PIPE SYSTEM

This is by far the most commonly used piping arrangement for wet heating systems, ranging from installations for very large commercial type buildings down to small domestic residential dwellings, due to the versatility of this arrangement.

As the name implies, the two pipe arrangement comprises two main heating distribution pipes in parallel as opposed to one on the single pipe system, see Figure 2.10. One of the pipes is a dedicated flow conveying hot water at boiler temperature to each heat emitter, whilst the other is a completely separate dedicated return that returns the water from each heat emitter, after it has transferred part of its heat, back to the boiler to be reheated.

Figure 2.10 Operating principles of a direct return two pipe heating system – non-condensing (condensing systems would employ different flow and return temperatures)

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The main advantage of the two pipe heating system over the single pipe heating system is that water is delivered to each heat emitter at the same temperature as it leaves the boiler, minus any piping heat losses. In turn, the water temperature that exits the heat emitters is the same temperature that is returned back to the boiler, again minus any pipework heat losses, whereas the single piping arrangement suffers from a progressive temperature drop across each piping circuit. The main disadvantage is that it is more costly and, with the direct return system illustrated in Figure 2.10, the supply and return lengths are unequal, which results in unbalanced flow and return flow rates.

This imbalance in the system flow rates is due to the tendency for the water to circulate through the heat emitters closest to the boiler, where the resistance to flow is at its lowest, at the expense of those furthest from the boiler where the resistance is at its greatest, resulting in the index circuit heat emitter becoming starved of water. This situation can be rectified by balancing the heating system using balancing devices, such as regulating valves and the lockshield return valves, on the heat emitters during the commissioning exercise on completion of the heating installation: this matter is discussed further later on in Chapter 25.

The direct return method of installing the two pipe heating system, where the flow and return pipes are in parallel to each other is usually simpler, as it is easier to install these two pipes side by side than it is to find alternative routes for them separately.

A variation of the two pipe system is the reverse return two pipe heating system as portrayed in Figure 2.11, which, through its flow path direction of both the flow and return circulating pipes, overcomes the problem of system flow disparity. It can be seen that the heating system depicted in Figure 2.11 is the same as that shown in Figure 2.10, the only difference being the arrangement of the main return pipe back to the boiler being routed in the opposite direction to that of the main flow pipe. This method of returning the water to the boiler creates an almost equal distance, and also an equal resistance to the flow of water to each heat emitter, thus making the balancing of the system flow much easier.

Figure 2.11 Operating principles of a reverse return two pipe heating system (non-condensing)

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The reverse return two pipe heating system is more desirable than the direct return system if capital costs can be justified and the building layout does not present complications to the pipe routes, as the heating system will remain in a balanced condition regardless of any tampering with the return regulating valves.

To illustrate the advantage of the reverse return piping system over the direct return arrangement of piping, two identical heating systems are shown in the isometric layout projection in Figure 2.12. Part (a) depicts the heating system with the piping arranged in a standard two pipe direct return layout where it can be seen that the resistance to the flow of heating water from the boiler and back again from heat emitter number 1 is much less than it is to any other of the heat emitters. The flow has to be encouraged by balancing the pipework system by making all the heat emitters have an equal resistance, so forcing the heating water to the index heat emitter and thus preventing the water short circuiting through the nearest heat emitter.

Figure 2.12 Isometric comparison of two pipe direct return and two pipe reverse return

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Part (b) shows the same heating system, but the pipework has been arranged as a two pipe reverse return configuration. In this example the resistance of the pipework encountered by the flow is the same to heat emitter number 1 as it is to heat emitter number 4, and all the other heat emitters in between. Therefore, providing that the piping system has been correctly sized, the circulation of the heating water is much simpler.

It can be argued that the additional capital cost of the two pipe reverse return heating system can be justified by the saving of the reduced time spent on balancing the piping network at the commissioning stage and, in turn, offsetting this initial capital cost.

MICRO BORE PIPING SYSTEM

All of the piping arrangements discussed so far have been equally suitable for large commercial heating systems and small domestic residential heating systems, but the micro bore heating system was developed in the 1960s specifically for domestic residential dwellings and small commercial properties, although it has been used successfully on larger building developments where it forms part of the overall larger heating scheme.

There have been a number of myths regarding the terms ‘micro bore’ and ‘mini bore’, some of them quite plausible, such as one of them being an open vented heating system and the other one being a sealed heating system, or that one uses imperial sized tubes while the other employs metric diameter tubes, this latter explanation being closest to being correct. The truth, however, is less romantic as they are in fact both proprietary names which have become acceptable to use to describe a heating system that utilises distribution pipes having diameters smaller than the 15 mm (½ inch), which are normally used on small bore heating systems. Mini bore was the first of these names to be used to describe water distribution pipes having diameters of ¼ inch and x215C_MinionPro-Regular_10n_000100 inch, but now micro bore has become generally adopted to describe heating systems employing tubes having diameters of 6 mm, 8 mm, 10 mm and 12 mm diameter.

The micro bore heating system differs fundamentally from any of the heating systems previously described. It employs components that are peculiar to the micro bore system and are not found on small bore heating systems that use conventional pipework and fittings common throughout the plumbing industry.

Figure 2.13 illustrates the basic operating principles of the micro bore heating system, which is still fundamentally a two pipe heating arrangement where the means of circulation is the same as any other form of piping arrangement, but the method of achieving that circulation differs from other piping layouts. The heated water is circulated from the boiler through conventional size pipes arranged as a pair of heating mains to strategically placed manifolds located fairly centrally between a group of heat emitters, this location being chosen to try to achieve reasonably equal micro bore branch pipe runs to each heat emitter that it serves. The manifold may be one of a variety of types, such as an inline multi­ple tee arrangement that incorporates a central blanking plate, or an end of line multiple reducer, or a number of other forms that are commercially available. The difference between this system and the other piping arrangements is that from the manifold, a separate dedicated micro bore flow and return pipe is extended radially and connected to each heat emitter individually, as shown. There is no limit to the number of manifolds employed, but each pair of micro bore flow and return pipes must be connected to the same manifold.

Figure 2.13 Operating principles of a micro bore heating system

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Heating systems employing conventional pipe sizes, i.e. 15 mm diameter and larger, are designed to have a flow velocity of approximately 1.0 m s−1: this has been found to be the most economic regarding the optimum velocity that will not cause noise to be generated or erosion to occur at sharp changes in direction. Micro bore piping arrangements do not conform to this convention and are designed to have flow velocities of 1.5 m s−1, which means that as the speed of the water being delivered to each heat emitter is one and a half times faster than that used on traditional piping arrangements, smaller diameter tubes may be used. This higher flow velocity through micro bore tubing is possible as there are no sharp changes in direction such as elbows, etc – and as the tubing is installed in one continuous length, using flexible coiled pipe of either soft fully annealed copper, or a barrier type plastic material, the same volume of water may be supplied to each heat emitter without any noise or erosion problems.

The following advantages are claimed for micro bore heating systems:

1. In existing buildings it is claimed to be easier to install, as fewer floorboards have to be removed when using micro bore tubing. Also, on new build properties the micro bore tubing can be installed by threading the flexible tube through holes pre-drilled in the floor joists from the ceiling below, allowing the floor to be laid earlier. This is possible as the micro bore tube – be it soft copper, a barrier thermoplastic material such as PEX, or polybutylene – is supplied in soft coil form that allows the tube to be threaded through in a similar way to electric cable. It should be noted that when working with soft fully annealed copper tube, although it is flexible when supplied in coil form, it quickly hardens if it is threaded through joists or below floorboards involving too many turns in direction and may need to be reheated to recover its original soft condition. Special tools are available to form neat labour-pulled bends and to straighten pipework that will be on show.

2. Double entry radiator valves are available and may be used, resulting in a cost saving compared to a pair of traditional single entry radiator valves that would normally be used. However, the type incorporating the provision to isolate the return connection as well as the flow connection should be selected to aid maintenance and allow the radiator to be removed without draining down the entire heating system.

3. A neater pipework installation is claimed as the piping may be hidden more easily in exposed situations, although, conversely, because the micro bore tubing is soft it may be more easily damaged, either accidentally or wilfully; therefore some form of protection should be considered if this potential exists. The pipework may also be passed through smaller openings or holes in the structure than conventional small bore tubing, which sometimes may be a deciding factor in specifying piping systems in existing buildings.

4. As the micro bore tubing is supplied and installed in one continuous length from the manifold to each heat emitter, the piping system requires fewer joints, which in turn means less possibility of leaks.

Manifolds

Figure 2.14 illustrates a typical inline multiple tee manifold that comprises a section of small bore tube, usually 22 mm or 28 mm diameter, which incorporates a centrally placed blank plate that divides the manifold into a separate flow zone and separate return zone, each with an equal number of compression type tees that convert the small bore tube into micro bore tube.

Figure 2.14 Inline multiple tee manifold

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The inline multiple tee type manifold is the most commonly used manifold of all the different variations commercially available and may be obtained with a varying number of connection tees to suit the application required, with any unused tees being blanked off. It is also where the flow velocity transforms from 1 m s−1 in the small bore tube to 1.5 m s−1 in the micro bore tube.

Figure 2.15 illustrates an alternative type of manifold that is in common use. It comprises either a capillary or compression linear fitting that is arranged to fit on the end of a small bore pipe that serves to divert the flow into a number of reduced size micro bore tubes. A separate linear type multiple reducing manifold is required for both the flow and return connections.

Figure 2.15 Linear multiple reducing manifold

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This type of manifold is restricted in physical size to a limited number of branch connections that are available to connect the micro bore tube into, where the inline multiple tee is not hindered.

Another form of micro bore manifold is shown in Figure 2.16; this type was available commercially for a period of time in a few sizes and was manufactured using cast iron for the body with non-ferrous compression pipe connections. There was an internal division plate arranged horizontally that formed two chambers, one for the flow and the other for the return, which meant that both the flow and return micro bore pipes serving each heat emitter were arranged almost one above the other on the same side of the manifold, thus avoiding having to cross over other micro bore pipes serving different heat emitters.

Figure 2.16 Alternative form of micro bore manifold

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The higher capital cost of this type of manifold limited its use to pipework that would be on show, as a very neat and professional appearance could be achieved when the manifold was fixed to a wall surface in a cupboard or service duct and the micro bore tubing dressed into the manifold pipe connections without any pipe crossovers.

Double Entry Radiator Valve

Figure 2.17 shows a cut through section of a double entry radiator valve illustrating the flow and return micro bore pipe connections and the application of a short cut piece of micro bore tubing used as a rigid insert to prevent the flow of water short circulating through the radiator. The flexible copper insert supplied with these radiator valves should be discarded as it has a tendency to lay flat on the bottom of the steel panel radiator welded seam, which could promote an electrolytic action between the two metals.

Figure 2.17 Detail and application of double entry radiator valve

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The flow enters the radiator through the valve flow connection and into the rigid copper insert where it circulates through the radiator and exits through the body of the valve on the outer side of the insert, as illustrated.

The double entry radiator valve is less costly than a pair of conventional radiator valves but can only be used on single panel radiators, or column type radiators. The construction of double panel radiators and radiators manufactured with back inlet connections prohibits the use of these double entry radiator valves.

TWO PIPE RADIAL SYSTEM

This is the most recent of piping arrangements forming a system of heat distribution that has been derived from the operating principles of the micro bore piping system, but it is not restricted to micro bore tubing or small bore tubing and incorporates components common to underfloor heating (see Chapter 6, Underfloor Heating).

Figure 2.18 illustrates the piping arrangement of the two pipe radial system. It can be seen that the distribution is very similar to that of the micro bore piping system except that the branch distribution piping may be 15 mm diameter or larger. The configuration of the manifolds used is very similar to those employed for underfloor heating, including the incorporation of individual isolating valves on both the separate flow and return manifolds.

Figure 2.18 Operating principles of two pipe radial system

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The method of delivering the heat is the same as for the micro bore system. The water is circulated by the pump through the common main flow and return pipes to each manifold located in a service cabinet or service duct, from where it is circulated through individual dedicated branch flow and returns to each heat emitter.

This piping arrangement has been more popular in Northern Europe than in the UK, but has been installed in a number of new housing developments in Britain over recent years. It is more suitable for installing in new build properties that have solid concrete floor constructions, but can be used in existing dwellings and buildings that have raised or ventilated timber floor construction if the property is undergoing a major refurbishment that can help to justify accommodating the building alteration costs.

Figure 2.19 shows a simple floor layout where the heating flow and return pipes are laid in the floor from the manifold and take a direct route below door openings to the heat emitters without any pipe joints, using large radius labour pulled bends.

Figure 2.19 Floor plan layout showing radial piping arrangement

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The piping material chosen for this application is normally either cross-linked polyethylene (PEX), polybutylene or polyethylene, all with an oxygen barrier incorporated within the pipe walls. These pipe materials have the qualities required for this particular application, such as flexibility, and are commercially available in long coiled lengths that allow the pipe to be laid in one continuous length without any intermediate joints. It is good practice to install these pipes inside a second conduit pipe that is directly embedded in the floor screed; see Figure 2.20. This has the advantage of permitting the carrier pipes to remain free to move through thermal movement inside the embedded conduit pipes, and facilitate the removal and replacement of the carrier pipes if necessary for maintenance.

Figure 2.20 Section through solid floor construction showing heating flow and return installed within conduit pipes

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This form of pipework installation makes for a neat and easy to install system, but makes it very difficult for any future alterations such as repositioning of heat emitters and property extensions.

Manufacturers of these systems produce a variety of ingenious components that form a transition from the embedded underfloor pipe to connect up to the heat emitters. Figure 2.21 shows an arrangement that utilises a junction box fitted into the floor screed that facilitates both the transition from a plastic pipe to a metallic pipe riser up to the heat emitter in the normal manner, and provides the access required to withdraw the carrier pipes from the conduit and replace with new if required.

Figure 2.21 Detail of junction box transition for heat emitter connection

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The conduit pipe may also be used to contain the flow and return carrier pipes embedded in wall chases or inside the cavity of dry-lined walls; see Figure 2.22. With this arrangement, a commercially available termination junction box, similar in appearance to an electrical power socket outlet cover plate, is fitted flush with the wall behind where the radiator is to be fixed, and the conduit pipes are fixed inside the depth of the walls with the carrier flow and return pipes installed inside them. The wall finish is then made good, and the flexible thermoplastic branch pipes connected to the conduit housed carrier pipes inside the termination junction box before the radiator is fixed to the wall. The flexible branch pipes can then be connected to the radiator valves, making for an aesthetically pleasing installation.

Figure 2.22 Flexible thermoplastic radiator connections from flush mounted termination box

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Thermoplastic pipes are extremely suitable for this application, their flexible nature enabling them to be connected from the termination junction box to the radiator.

In newly constructed buildings the radial piping arrangement has a number of advantages, primarily reduced on-site installation time, meaning the time for which other trades are prevented from working whilst the piping system is being installed is reduced. This reduction in programme time results in a lower cost of construction to the developer. The radial piping arrangement also results in a neat, unobtrusive installation. However, the price to pay for these construction phase advantages comes later when the building owner wishes to extend or alter the building, as the resulting pipework alterations involve considerably more work for the builders.

HYBRID (MIXED) SYSTEMS

Figure 2.23 illustrates a somewhat exaggerated example of a mixed piping system arrangement demonstrating the possibilities that are available if the building layout, together with the client’s needs, dictate it. It is equally suitable for large or small buildings alike.

Figure 2.23 Hybrid system of mixed piping arrangements

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Hybrid systems need not just be a mixture of different piping arrangements; they can also be a combination of temperature/pressure configurations.

Figure 2.24 illustrates a hybrid system of sealed heating where there is a mixture of piping arrangements and a mixture of operating temperatures. This form of heating is more common in other parts of Europe than it is in the UK. The system is designed to operate as a medium pressure heating scheme with the water flow temperature leaving the boiler at a temperature of 100–120°C, making it unsuitable for domestic residential dwellings. The high temperature flow water first travels via a single pipe system to pass through a series of low surface temperature radiators or convectors where the high temperature of the water can be exploited to the full, permitting physically smaller heat emitters to be selected. When the high temperature of the flow has been exhausted and the flow temperature falls below 100°C, the water then passes on to a conventional two pipe system, still operating at medium pressure, but permitting stand­ard panel or column type radiators to be employed that have been selected and sized in the same way as for a conventional low pressure heating system. The water is then returned to the boiler at a normal return temperature of 71°C.

Figure 2.24 Hybrid system of mixed piping and temperature arrangement (not suitable for domestic residential dwellings)

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This hybrid arrangement has obvious advantages if the building criterion favours it.

OPEN VENTED HEATING SYSTEMS

All the hydronic heating systems depicted so far in this text, with the exception of the sealed heating system illustrated in Figure 2.24, are described as ‘open vented heating systems’, which is the term used to describe hydronic systems of heating subjected to hydrostatic pressure only. This hydrostatic pressure is determined by the physical height that the water in the feed and expansion cistern is exerting on the rest of the heating system. Heating systems in the past in the UK have traditionally – but not exclusively – been of the open vented type, and the reason why the drawings used in this chapter of hydronic heating systems are of the open vented type is to show the need for, and method of, accommodating the expansion of the water caused by the application of heat.

One of the main features of an open vented system, and the foremost component that makes it an open vented system of hydronic heating, is the inclusion of the feed and expansion cistern.

The function of the feed and expansion cistern may be explained in three ways:

1. The initial purpose of the feed and expansion cistern is to serve as a header tank to enable the heating system to be filled up with water and then to continue to perform as a reservoir, where any water losses due to evaporation may be automatically replenished. The feed and expansion cistern is the automatic point of introduction of water into the heating system, both on the initial fill and any subsequent fills that may be required during maintenance activities. This is the feed part of the feed and expansion cistern that functions before the heating system becomes operational and is only required when water is to be added to the system.

2. The second function of the feed and expansion cistern is to serve as a means of catering for the volumetric increase in water caused by the expansion of the water that occurs when the heating system boiler is operating at its design temperature, without causing any pressurisation of the heating system. This is the expansion part of the feed and expansion cistern and is performed continually during its normal operation.

3. The feed and expansion cistern also has a third function, which is equally important but not immediately obvious as the first two points. This third function is one of safety; it serves as a cooling tank in the event of the boiler overheating and high temperature water/steam being discharged into it from the open vent pipe terminating over the top of the feed and expansion cistern. This high temperature water/steam mixes with the water already in the feed and expansion cistern, which is also at a high temperature, but also mixes with any incoming cooler make-up water to replace evaporation losses before returning it back into the heating system to keep the system and boiler wet and thus prevents the boiler becoming dangerous. Because of this safety aspect, it is imperative that the feed and expansion cistern is correctly sized and that it is manufactured from materials capable of withstanding the high temperatures that could exist when this condition occurs. This third function is not always fully appreciated as its importance does not become apparent until something goes wrong with the heating system. If it has not been designed correctly a failure to the feed and expansion cistern could occur compounding a dangerous situation and creating a potentially catastrophic condition.

FEED AND EXPANSION (F&E) CISTERN

When water is heated, as it is in a hydronic heating system, the rising temperature will cause the water to expand. This increase in volume will have to be catered for, otherwise the heating system will become highly pressurised, to a point where it is in imminent danger of fracturing at its weakest part. In the open vented heating system this increase in water volume is accommodated by the feed and expansion cistern.

Water is at its maximum density at a temperature of 4.4°C; any change in temperature from this point, be it higher or lower, will result in an increase in its volume. If the temperature of water is raised from 4.4°C to 100°C, it will increase in volume by 4.2%, or c02ue001 of its original volume. Therefore the feed and expansion cistern must be sized to accommodate not less than c02ue002 of the heating system volume of cold water, plus the water in the lower part of the cistern required to allow the float valve to operate, a minimum depth of 100 mm and a safety factor.

These requirements are satisfied by allowing a volumetric increase of 5%, or c02ue003 of the heating system’s water content when cold and is the figure recommended by BS 5449, and other authoritative engineering guides. This increase in volume should be allowed for above the normal cold water level and below the warning pipe/overflow level as the water should expand into the feed and expansion cistern via the cold feed, but should not overflow through the warning pipe. When the heating system is turned off, the water will cool down and contract back into the heating system by the way it entered, the cold feed.

Table 2.3 gives the minimum recommended nominal capacities for feed and expansion cisterns, together with their service connections, but it should be emphasised that these should only be used as a guide and the final selected size should be calculated, see Box 2.1.

Table 2.3 Recommended minimum capacities for F&E cisterns and connections

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BOX 2.1 Example calculation of F&E cistern size

Assuming that the F&E cistern measures 500 × 300 mm and that it contains water up to a height of 100 mm, this would equal 15 litres of water when the system is cold.

15 + 4.65 litres = 19.65 litres of water actual capacity.

Assuming an equal volume of 15 litres above the acceptance volume water line then:

15 + 19.65 litres = 34.65 litres of water nominal capacity

Therefore, capacity of the feed and expansion cistern required =

The feed and expansion cistern should conform to the requirements of the Water Regulations exactly as for the main cold water storage cistern, as it is supplied with cold water from the incoming rising main. Figure 2.25 illustrates the general arrangement of the feed and expansion cistern detailing the main points. The construction materials should be suitable for their uses; particularly for the high water temperature that could exist if nearby boiling water is discharged into it during a boiler/system malfunction. If the cistern is not self-supporting, a solid